The present invention relates to information storage media represented by compact disks (CD), laser disks (LD), compact disk-type read-only memory disks (CD-ROM), and digital versatile disk-type video disks (DVD), from which previously recorded information can be reproduced by using converged light; or optical disks (MO, OD, MD), from which previously recorded information can be reproduced by using converged light and in which previously recorded information can be rewritten by using converged light; and once-write-type CD-ROM disks (CD-R) among, which permit writing of information. The present invention also relates to information reproducing apparatuses for reproducing information from information storage media, writing information to information storage media, or rewriting previously recorded information to information storage media.
On an information storage medium wherein optical characteristics, magnetic characteristics, or the shape in the local area of a recording film surface varies by using converged light to form small record marks so information is written or rewritten, pre-grooves for decreasing track pitch are formed. Track pitch determines radial intervals of record marks. The pre-grooves are arranged in groove shapes on the recording film surface on which record marks are to be formed.
During information recording, a pre-groove (continuous groove) is traced by using a converged beam spot produced by a recording/reproducing apparatus, and record marks are successively formed in the pre-groove or on a land portion between pre-grooves.
The pre-groove is basically formed in a continuous spiral shape on a recording surface of the information storage medium. However, if microscopically viewed, the pre-groove is interrupted at boundaries of sectors, by the sectors which are very finely divided along the pre-groove. Pre-pits having very small concave shapes are formed at the interrupt portions between the pre-grooves. For example, the pre-pits have information representing the sector numbers assigned to the individual sectors. In many cases, the pre-pits also have sync codes representing a reference clock at the time of information reproduction.
Suppose that a diameter of a reproducing beam spot having central intensity e.sup.-2 is Ws, the wavelength of the reproduced light is .lambda., the refractive index of a transparent plastic base plate of the information storage is n, the width of the pre-groove is Wt, the depth of the pre-groove is dt, the width of the pre-pit is Wp, and the depth of the pre-pit is dp. When a laser beam emitted from an optical head of the information reproducing apparatus is converged on the information storage medium through an objective lens, and a pre-pit signal is reproduced based on the variation in the amount of light which is included in reflected light from the information storage medium and has passed back through the objective lens, the maximum reproduction signal is obtained when the following condition is satisfied: EQU Wp.apprxeq.Ws/3,dt.apprxeq..lambda./(4n) (A)
When a push-pull method is employed, wherein light, which has been reflected by the information storage medium and has passed back through the objective lens, is divided into two components by wave front division with respect to a straight line including a center axis and a difference between the two components is detected to find a tracking error, the maximum tracking error signal is obtained when the following condition is satisfied: EQU Wt.apprxeq.Ws/2,dp.apprxeq..lambda./(8n) (B)
If the width and depth of the pre-pit are plotted on the abscissa and a detection signal is plotted on the ordinate of a graph, a maximum value is obtained when formula (B) is satisfied. Thus, even if the width and depth of the pre-pit vary slightly near the optimal values, the detection signal does not vary substantially. In contrast, when the width and depth of the pre-pit depart from optimal values, the detection signal varies greatly even if the width and depth only slightly vary.
However, for reasons relating to the manufacture described below, it is difficult to form both the pre-pit and pre-groove with optimal shapes. The following problems arise.
1) Either the pre-pit shape or the pre-groove shape is less regarded during manufacture, and the detection signal of the less regarded one decreases and the precision of detection deteriorates.
2) Since either the pre-pit shape or the pre-groove shape is not formed with the optimal shape, the detection signal varies very sensitively to a slight variation in depth or width. As a result, detection signals vary greatly from manufacturing lot to manufacturing lot of information storage media, and the manufacturing yield of information storage media decreases considerably. Since output signals depart from maximum values if the optimal conditions are not met, as described above, detection signals vary greatly in relation to the variation in width and depth.
3) The amount of exposure light on a primary disk of a primary disk recording apparatus for information storage media needs to vary in at least three levels (three-level exposure amount including zero-level). It is very difficult to vary the exposure amount stably in multiple levels and to ensure high exposure precision in each level.
4) As a result, there is difficulty in manufacturing information storage media, manufacturing yield decreases, and the manufacturing cost of information storage media increases.
The structure of the conventional information storage medium as well as the method of manufacturing the same will now be described, and some problems in the prior art will be explained in detail.
The information storage medium is manufactured by the steps (a) to (e):
a) forming a primary disk,
b) forming a stamper by electroforming plating,
c) forming a plastic base plate by injection molding,
d) forming a recording film by deposition, and
e) bonding.
Pre-grooves and pre-pits are first formed on the recording film by step (a), forming the primary disk.
The most important problem in this case is that the pre-groove and pre-pit have different optimal depths (dt.apprxeq..lambda./(4n); dp.apprxeq..lambda./(8n)).
In order to meet the two depth conditions, the use of a photoresist layer having a two-layer structure was proposed. However, high-quality recording cannot be achieved due to cross-talk between the two layers of the photoresist in the exposure/development step.
Even when two layers, a metal film of, e.g. Te (tellurium) and a photoresist layer, are used, this structure cannot practically be used because of rims of Te near the holes.
Considering the above, in order to stably form pre-grooves and pre-pits with high productivity, it is essential to use a single-layer photoresist in forming a primary disk. This condition is unchanged even now.
Under such circumstances, in the prior art, the amount of exposure light on the photoresist layer is controlled in the step of exposing the primary disk, thereby varying the depths of the pre-groove and pre-pit.
FIG. 1 shows the shapes and dimensions of the thus obtained pre-grooves and pre-pits on the primary disk. If the transfer efficiency in the steps (b), (c), and (d) in the primary disk manufacturing process is 100%, the shapes and dimensions of the pre-grooves and pre-pits formed on the recording film of the information storage medium agree with those of the pre-grooves and pre-pits formed on the primary disk.
Referring to FIG. 1, the depth dt of the pre-groove 11 is about .lambda./(8n). The reason is that the laser beam does not reach the bottom face of the photoresist layer (i.e. the position of a glass plate) because the light amount of the laser beam is set at a low level, despite the thickness dr of the photoresist layer being set at .lambda./(4n). In addition, since the exposure amount is small, the width Wt of the pre-groove 11 is very narrow.
By contrast, the depth dp of the pre-pit 12 is about .lambda./(4n) which is equal to the thickness dr of the photoresist layer, because the light amount of the laser beam is increased and the photoresist layer can be exposed even to the bottom face thereof. The width Wp of the pre-pit 12 is, as mentioned above, Wp.apprxeq.Ws/3.
Consequently, the conventional structure of the information storage medium, as shown in FIG. 2, has the following problems:
Although the shape of the pre-pit 12 is determined to obtain a maximum reproduction signal,
the width Wt of the pre-groove 11 is very small and greatly differs from an ideal value, Wt.apprxeq.Ws/2. Thus, a tracking error detection signal obtained by a push-pull method decreases greatly. Because of the deficiency of the detection signal, the information reproducing apparatus cannot stably control the tracking, and tracking errors often arise.
If the conditions for manufacture are the same, the depth dt of the pre-groove 11 is dt.apprxeq..lambda./(8n). However, the depth dt greatly varies due to a slight variation in conditions for development in a developing step (e.g., concentration of developing liquid, developing time, room temperature at the time of development). As a result, amplitudes of tracking error detection signals vary greatly from manufacturing lot from manufacturing lot. When information is to be reproduced by the information reproducing apparatus, a tracking error control circuit cannot be used in the case of a specific information storage medium since it oscillates due to an excessively large tracking error detection signal. In the case of another information storage medium, the tracking error detection signal is too large to stably correct a tracking error, and tracking errors often arise.